Optical triangulation methods are used in a variety of ways in order to contactlessly measure the surfaces or the entire geometrical shape of three-dimensional objects. In general, triangulation is a geometrical method of optical distance measurement by angle measurement within triangles. The calculation is in this case carried out by means of trigonometric functions. In general, distinction is made between passive and active triangulation methods. In contrast to passive methods, active methods have a projector unit which projects structured light onto the surface of an object. The projector unit may be a projector or a laser, or a laser diode. According to the prior art, light is projected onto the surface of an object to be measured. The scattered light is subsequently recorded at a fixed angle, the triangulation angle, by means of a camera and analyzed. The connecting line between the light source and the camera, and the two light beams from and to the object to be measured span a triangle, so that, with a known distance between the light source and the camera and known beam directions, the distance between the camera and the object can be determined.
For three-dimensional measurement by means of color-coded triangulation, a pattern of colored stripes, which is produced for example by means of a transparency, is typically projected with a predetermined beam direction onto the object to be measured. It is advantageous that spatial positions in the projector are represented with color coding on the object surface. The colored scattered light is subsequently analyzed by means of a camera at a fixed angle. Because of the curved shape of the object surface, the colored stripes experience a position-dependent phase shift, from which the shape of the surface can ultimately be determined. However, the colored stripes in the image of the scattered light are subject to brightness modulations that result from locally color-dependent absorption and reflectivity on the object surface. Furthermore, a superposition with the usually colored light from the surroundings always takes place. Thus, for example, there may be a shift of the colors in the color space in the image or the individual colors may become difficult to identify because of the loss of brightness. According to the prior art, an attempt is made to compensate for this by HDR cameras. Particularly for medical applications, however, it has not yet been possible to use this technology because of the rapid object movements.
On the camera sides, image sensors with an upstream Bayer sensor are used according to the prior art. The Bayer sensor in this case has three sensitive spectral ranges, which usually lie in the blue, green and red. The colored light of the image of the object surface can therefore be spectrally filtered according to color before it strikes the photosensitive surfaces of the image sensor. However, the arrangement for determining the color is very inaccurate since, as is known, crosstalk of the colors can occur in a Bayer sensor. As an alternative to cameras with a Bayer sensor, it is also possible to use 3-chip cameras, the color separation of which is somewhat better in comparison. The unsharp selection or separation of the colors leads as a consequence to measurement discrepancies in the determination of the three-dimensional shape of the object to be measured. In the case of a red projected stripe, for example, a green signal may even result because of an overlap of the sensitive regions of the Bayer sensor. In the case of object surfaces which have a large contrast dynamic range, this leads to an erroneous evaluation of the color of a stripe and therefore to missing surface regions, which may need to be filled in by multiple scans with different triangulation angles.